In recent years, high-density packing has been demanded for circuit boards of electronic equipment to achieve size reduction, weight reduction, higher performance, and higher speed for the electronic equipment. For this reason, mounting structures have been reduced in size and thickness and become faster with an increased number of terminals. A mounting structure contains electronic components and chip components to be mounted on circuit boards, semiconductor elements in a BGA (Ball Grid Array), semiconductor elements in a LGA (Land Grid Array) where flat electrode pads are arranged in a grid-like fashion instead of solder balls of BGA, and semiconductor elements in a CSP (Chip Size Package). Consequently, mounting structures have decreased in mechanical strength and the number of mounting structures susceptible to an applied mechanical stress and a temperature change has increased.
In a typical method of mounting a semiconductor element on a circuit board, the electrodes of the semiconductor element are brought into contact with predetermined positions on the circuit board, a solder material or a conductive adhesive is supplied between electrodes to be connected, the semiconductor element and the circuit board are placed into a reflow furnace and the like to join the electrodes of the semiconductor element and the circuit board, and a gap between the semiconductor element and the circuit board is molded with a underfill resin material together with portions around joints concurrently with or before or after the joining operation.
The semiconductor element is bonded and fixed to the circuit board through the resin molding, so that the joints can be protected with high reliability even when the semiconductor element is exposed to a heat cycle or a hot and humid environment after the resin molding.
Further, in the resin molding, a fillet formed around the semiconductor element considerably affects the reliability of the semiconductor element and thus it has been requested to stably form fillets.
FIG. 8A shows a mounting structure in which semiconductor elements 1b and 1b are surface-mounted on a circuit board 2. FIG. 8B shows a state before underfill resin 3 is applied. FIG. 8C shows a state in which the underfill resin 3 has been applied between the semiconductor element 1b and the circuit board 2 and between the semiconductor element 1b and the circuit board 2.
FIG. 9A shows the circuit board 2 before the semiconductor elements 1b and 1b are mounted. FIG. 9B is a sectional view taken along line X-X of FIG. 9A. The overall mounting surface of the circuit board 2 is formed as a ground pattern 5, except for lands 4 on which the semiconductor elements 1b and 1b are mounted.
In this case, as indicated by arrows in FIG. 8A, the low-viscosity underfill resin 3 is injected in small quantities by a syringe and the like from locations around the semiconductor elements 1b and 1b according to the current state of the art.
Patent Document 1 describes a technique in which dams are provided around the semiconductor elements 1b and 1b to prevent the injected underfill resin 3 from flowing outside.    Patent Document 1: Japanese Patent Laid-Open No. 2006-237367